This invention generally relates to exhaust systems of internal combustion engines, and more particularly to an engine exhaust cleaning device.
Emissions of engine exhausts containing CO as well as HC and NO.sub.x which are considered as sources of photochemical smogs are now under severe restrictions. As is well known, CO is generated by incomplete combustion of carbons which occurs due to lack of oxygen. This is to say, the amount of CO in the engine exhausts can be reduced by burning in the engine a lean air-fuel mixture or a combustible mixture having a high air ratio. In this connection, it has been proposed to provide a riser or the like in the exhaust pipe of the engine to heat a lean air-fuel ratio mixture for the purpose of attaining complete combustion of the fuel without misfiring to reduce the amount of CO in the engine exhausts.
It has also been proposed to provide a reactor upstream of an exhaust pipe for accelerating oxidation reactions of CO as well as HC which has remained unburned due to low temperatures prevailing in the vicinity of combustion chamber walls, thereby converting them into unharmful forms of H.sub.2 O and CO.sub.2.
On the other hand, if the combustion in the combustion chambers of the engine is effected at an excessively high level, a large amount of NO is produced, changing into NO.sub.2 upon contact with air. The combustion temperature may be lowered by introducing an inert gas into the combustion chambers for heat absorption. In this connection, it has already been proposed to recirculate a portion of the exhaust gas from the exhaust pipe to the intake pipe of the engine by the so-called exhaust gas recirculating (EGR) system.
However, the existing EGR systems are usually adapted to recirculate a portion of exhaust gas of each cylinder in a multi-cylinder internal combustion engine and therefore have a drawback in that the EGR rate (amount of recirculated exhaust gas/ (amount of recirculated exhaust gas + intake air amount)) becomes smaller with a cylinder of a larger piston displacement, allowing temperature rises and producing NO.sub.x in an increased degree. In order to reduce the amount of NO.sub.x efficiently without deteriorating the performance of the vehicle, it is desirable to recirculate the exhaust gases in proportion to the amount of intake air of the engine. However, the proportional EGR has thus far been possible only with a complicate correcting or adjusting device.
With the conventional devices of this nature, the unrecirculated portion of the exhaust gases which have been deprived of heat in a riser or the like is released into the air without undergoing sufficient reactions in the reactor, thus precluding complete cleaning of the exhaust gases,